A dye sublimation thermal transfer method (a thermal sublimation-type transfer method) is a method that comprises providing a thermal transfer film comprising a base material bearing a dye layer on its surface, the dye layer having been formed using a solution or dispersion of a sublimable dye in a binder resin, superimposing the thermal transfer film on an image receiving film, applying energy according to image information to a heating device such as a thermal head to transfer the sublimable dye, contained in the dye layer in the thermal transfer film, onto the image receiving film and thus to form an image.
The thermal sublimation-type transfer method is advantageous, for example, in that, since the amount of the dye to be transferred can be regulated dot by dot by regulating the amount of energy applied to the thermal transfer film, excellent gradational images can be formed and, at the same time, characters, symbols and the like can simply be formed.
In this thermal transfer method, high-quality images comparable with silver salt photographs can be formed. This has led to a very strong demand for the prevention of a deterioration in the quality of images due to factors such as light, heat and humidity, and, in order to satisfy this demand, the development of various sublimable dyes for image storage stability improvement purposes has been made.
For example, Japanese Patent No. 3013137 (patent document 1) and Japanese Patent No. 3078308 (patent document 2) disclose, as a coloring matter, for thermal dye transfer recording, possessing excellent transferability and storage stability, an azomethine compound having a structure comprising a 1H-pyrazolo[5,1-C][1,2,4]triazole ring as a coupler and a pyridyl group bonded to the coupler through a nitrogen atom. Further, Japanese Patent No. 2840901 (patent document 3) discloses an azomethine compound having a structure comprising a 1H-pyrazolo[1,5-b][1,2,4]triazole ring as a coupler and a phenylamino group bonded to the coupler through a nitrogen atom. Furthermore, Japanese Patent Application Laid-Open No. 239367/1993 (patent document 4) discloses an azomethine compound having a structure, which is a combination of both the above structures, comprising a pyridyl group bonded to a 1H-pyrazolo[1,5-b][1,2,4]triazole ring coupler through a nitrogen atom.
The azomethine coloring matters disclosed in Japanese Patent No. 3013137 and Japanese Patent No. 3078308 have high lightfastenss but pose a problem of cost because the 1H-pyrazolo[5,1-C][1,2,4]triazole ring functions as the coupler. Further, the azomethine coloring matter, which is described in Japanese Patent No. 2840901 and uses the 1H-pyrazolo[1,5-b][1,2,4]triazole ring compound as the starting material coupler, can be advantageously produced at relatively low cost but sometimes suffers from unsatisfactory lightfastness.
On the other hand, the coloring matter, which is described in Japanese Patent Application Laid-Open No. 239367/1993 and comprises a combination of the 1H-pyrazolo[1,5-b][1,2,4]triazole ring coupler with the pyridyl group, can be advantageously produced at low cost and, at the same time, has high lightfastness. In particular, compounds 9, 10, 11, 22, and 112, which are proposed in Japanese Patent Application Laid-Open No. 239367/1993 and have a phenyl group introduced as substituent R6 of the 1H-pyrazolo[1,5-b][1,2,4]triazole ring, are advantageous in that the color tone of the coloring matter is close to a required color reproduction region. Compounds described in Japanese Patent Application Laid-Open No. 239367/1993, especially compounds with an unsubstituted pyridyl group introduced thereinto as substituent R6 of the triazole ring, are advantageous in production cost and lightfastness but suffer from a low degree of conversion in the coupling reaction and provide a yield of about 20% as described also in Japanese Patent Application Laid-Open No. 239367/1993.
Further, in addition to the development of yellow, magenta, and cyan dyes, various research and development have also been made on a combination of dyes of these colors. For example, Japanese Patent Application Laid-Open No. 87309/2008 (patent document 5) proposes a thermal transfer sheet that can yield printed matters having various excellent fastness properties such as high lightfastness and less susceptibility to catalytic fading through a combination of specific yellow dye coloring matter, magenta dye coloring matter, and cyan dye coloring matter.